Modifying atrioventricular delay based on activation times

ABSTRACT

Methods and/or devices may be configured to monitor ventricular activation times and modify an atrioventricular delay (AV delay) based on the monitored ventricular activation times. Further, the methods and/or devices may determine whether the AV delay should be modified based on the measured activation times before modifying the AV delay.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.13/744,038 (now allowed), filed Jan. 17, 2013 entitled “MODIFYINGATRIOVENTRICULAR DELAY BASED ON ACTIVATION TIMES” and also claims thebenefit of U.S. Provisional Patent Application Ser. No. 61/588,924 filed20 Jan. 2012, entitled “MODIFYING ATRIOVENTRICULAR DELAY BASED ONACTIVATION TIMES,” which is incorporated herein by reference in itsentirety.

BACKGROUND

The disclosure herein relates to methods and devices for modifyingatrioventricular delays based on activation times within cardiac tissueto, e.g., maintain effective pacing therapy.

In the normal human heart, the sinus node, generally located near thejunction of the superior vena cava and the right atrium, constitutes theprimary natural pacemaker initiating rhythmic electrical excitation ofthe heart chambers. The cardiac impulse arising from the sinus node istransmitted to the two atrial chambers causing a depolarization and theresulting atrial chamber contractions. The excitation pulse is furthertransmitted to and through the ventricles via the atrioventricular (AV)node and a ventricular conduction system causing a depolarization andthe resulting ventricular chamber contractions.

Disruption of this natural pacemaking and conduction system as a resultof aging or disease can be treated by artificial cardiac pacing. Forexample, one or more heart chambers may be electrically paced dependingon the location and severity of the conduction disorder. In addition,cardiac pacing for ventricular dyschrony, often referred to as cardiacresynchronization therapy (CRT), may include pacing one or bothventricles before normal conduction through the AV node depolarizes theventricles.

Implantable medical devices (IMDs) are capable of utilizing pacingtherapies, such as CRT, to maintain hemodynamic benefits to patients.Pacing therapy may be delivered from an implantable generator, through alead, and into the patient's heart. Basic programmable pacing parametersinclude atrioventricular delay (AV delay), left ventricle to rightventricle delay (VV delay), pacing amplitude, pacing rate, pulseduration, and pacing pathway or vector (e.g., bipolar such as a lead tipelectrode to a lead ring electrode, etc. or unipolar such as a lead tipelectrode to IMD casing, or housing), which all may be configured toensure effective therapy to the patient.

For some patients suffering from heart failure and intraventricularconduction delays due to, e.g., left bundle branch block, right bundlebranch block, the delivery of CRT can occur due to a single ventricularpacing stimulus by pre-exciting the ventricle with conduction delay.Such a stimulus must be properly timed relative to intrinsicdepolarization of the other, non-delayed ventricle. This phenomenon maybe referred to herein as “fusion pacing” since ventricular activationfrom a pacing stimulus fuses or merges with ventricular activation fromintrinsic conduction. When the ventricular pacing stimulus is properlytimed a desired ventricular resynchronization results with a minimum ofpacing energy, thereby extending the operating life of an implantablepulse generator (e.g., an implantable cardioverter-defibrillator,pacemaker, and the like). Moreover, in some cases a more effective orphysiologic form of CRT delivery can be achieved since the system andmethods herein utilize a portion of intrinsic activation, which can besuperior to an entirely evoked (e.g., paced) form of CRT. Fusion pacingmay also be referred to herein as left ventricle-only pacing or rightventricle-only pacing.

One method of fusion pacing, or left ventricle-only pacing, includespacing the left ventricle at an appropriate time to achieve fusion of apaced wavefront with an intrinsic depolarization of the right ventricle.One method of fusion pacing, or right ventricle-only pacing, includespacing the right ventricle at an appropriate time to achieve fusion of apaced wavefront with an intrinsic depolarization of the left ventricle.Such a CRT method may reduce device power output relative tobiventricular pacing and may improve hemodynamics, especially at lowerheart rates.

One specific parameter that may be used by an IMD to deliver cardiactherapy (e.g., CRT such as left ventricular fusion pacing) is anatrioventricular delay (AV delay), which may generally be described as aprogrammable value representing a time period between atrial electricalactivity, whether intrinsic (e.g., natural) or paced, and the deliveryof ventricular pacing. The optimal value of the AV delay has generallybeen defined as a delay that produces the maximum stroke volume for afixed heart rate or the maximum cardiac output for a sinus node drivenheart rate.

To optimize or adjust the AV delay, a cardiac therapy device such as anIMD may measure a patient's intrinsic AV conduction time. A patient'sintrinsic AV conduction time is the time between an intrinsic atrialevent (e.g., depolarization of the right atrium) and an intrinsicventricular event (e.g., depolarization of the right ventricle). As usedherein, an “intrinsic” event or conduction is one that occurs or isconducted naturally (e.g., an intrinsic ventricular event is an eventtriggered by electrical activity transmitted across the AV node of theheart from the atria to the ventricles, etc.). A cardiac therapy devicemay periodically measure a patient's intrinsic AV conduction time, orinterval, and adjust the AV delay in response to the measured intrinsicAV conduction time, e.g., to optimize cardiac functionality.

For example, a CRT algorithm (e.g., performed by an IMD) may measure apatient's intrinsic AV conduction time once every minute by forcingdelays used for ventricular pacing (e.g., paced AV delay, sensed AVdelay, etc.) to long values (e.g., 300 milliseconds (ms), 350 ms, etc.).Conventionally, the intrinsic AV conduction time measurement has beenperformed periodically (e.g., every 60 seconds) so that the CRTalgorithm can adapt to changes in the patient's intrinsic AV conductiontime.

In other words, CRT algorithms may temporarily suspend, or interrupt,pacing therapy for one or more heartbeats to measure a patient'sintrinsic AV conduction time for use in modifying or adjusting (e.g.,optimizing) one or more pacing parameters such as AV delay.

SUMMARY

One exemplary implantable medical device operable for delivery ofcardiac therapy to a patient includes a therapy delivery module, asensing module, and a control module coupled to the therapy deliverymodule and to the sensing module. The therapy delivery module may beconfigured to deliver pacing therapy to either the left ventricle or theright ventricle of a patient's heart using at least one electrode. Thesensing module may be configured to sense electrical activity of thepatient's heart (e.g., electrical activity of the right or leftventricle, far-field electrical activity of the right or left ventricle,near-field electrical activity of the right or left ventricle, etc.)using at least one electrode. The control module may be configured tocontrol the delivery of pacing therapy to either the left ventricle orthe right ventricle of a patient's heart based on an AV delay (where thepacing therapy is delivered over a plurality of heartbeats) and senseelectrical activity of the patient's heart using the sensing moduleduring the delivery of the pacing therapy. The control module may befurther configured to measure a ventricular activation time for each ofthe plurality of heartbeats between the delivery of pacing stimulus ofthe pacing therapy and at least one selected fiducial point of thesensed electrical activity (e.g., a maximum negative slope of thefar-field electrical activity of the right ventricle of the patient'sheart, a maximum value of the near-field electrical activity of theright ventricle of the patient's heart, etc.) resulting from at leastone of the delivered pacing stimulus of the pacing therapy and anintrinsic conduction of the patient's heart. The control module may befurther configured to modify the AV delay for use in delivering pacingtherapy based on the measured ventricular activation times if one ormore of the measured ventricular activation times are greater than orless than a predetermined reference activation time by a selectedthreshold value (e.g., if a first selected number of the measuredventricular activation times are greater than or less than thepredetermined reference activation time by the selected threshold valueover a second selected number of heartbeats).

One exemplary method for use in an implantable medical device operablefor delivery of cardiac therapy to a patient includes delivering pacingtherapy to either the left ventricle or the right ventricle of apatient's heart using at least one electrode based on an AV delay usingan implantable medical device (e.g., where the pacing therapy isdelivered over a plurality of heartbeats) and sensing electricalactivity of the patient's heart (e.g., electrical activity of the rightor left ventricle, far-field electrical activity of the right or leftventricle, near-field electrical activity of the right or leftventricle, etc.) using at least one electrode of the implantable medicaldevice during the delivery of the pacing therapy. The exemplary methodmay further include measuring a ventricular activation time for each ofthe plurality of heartbeats between the delivery of pacing stimulus ofthe pacing therapy and at least one selected fiducial point of thesensed electrical activity (e.g., a maximum negative slope of thefar-field electrical activity of the right ventricle of the patient'sheart, a maximum value of the near-field electrical activity of theright ventricle of the patient's heart, etc.) resulting from at leastone of the delivered pacing stimulus of the pacing therapy and anintrinsic conduction of the patient's heart. The exemplary method mayfurther include modifying the AV delay for use in delivering pacingtherapy based on the measured ventricular activation times if one ormore of the measured ventricular activation times are greater than orless than a predetermined reference activation time by a selectedthreshold value (e.g., if a first selected number of the measuredventricular activation times are greater than or less than thepredetermined reference activation time by the selected threshold valueover a second selected number of heartbeats).

In one or more exemplary devices and methods, modifying the AV delay foruse in delivering pacing therapy based on the measured ventricularactivation times may include setting the AV delay to the last AV delayplus the last measured ventricular activation time minus thepredetermined reference activation time or setting the AV delay toeither a mode or a median of a selected number of previous AV delaysplus either a mode or median of the selected number of ventricularactivation time differences. The ventricular activation time differencesmay be the measured ventricular activation times minus the predeterminedreference activation time.

In one or more exemplary devices and methods, modifying the AV delay foruse in delivering pacing therapy based on the measured ventricularactivation times may include modifying the AV delay for use indelivering pacing therapy if a standard deviation of a first selectednumber of the activation time differences are less than a selectedvariability threshold value over a second selected number of heartbeats,wherein the ventricular activation time differences are the measuredventricular activation times minus the predetermined referenceactivation time.

In one or more exemplary methods and devices, the exemplary methods mayfurther include and the control module of the exemplary devices may befurther configured to execute delivering bi-ventricular pacing therapyto the patient's heart using the implantable medical device if themodified AV delay is greater than a selected limit value.

The above summary is not intended to describe each embodiment or everyimplementation of the present disclosure. A more complete understandingwill become apparent and appreciated by referring to the followingdetailed description and claims taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an exemplary system including an exemplaryimplantable medical device (IMD).

FIG. 2 is a diagram of the exemplary IMD of FIG. 1.

FIG. 3 is a block diagram of an exemplary IMD, e.g., the IMD of FIGS.1-2.

FIG. 4 is a graph depicting electrograms of different sensing vectorsfor use in measuring right ventricular activation time.

FIG. 5 is a general flow chart of an exemplary method of modifyingatrioventricular delay (AV delay) based on ventricular activation times,e.g., using the IMDs of FIGS. 1-3.

FIG. 6 is a flow chart of an exemplary method of modifying AV delaybased on right ventricular activation times.

FIG. 7 depicts three exemplary right ventricular activation timecalculations.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

In the following detailed description of illustrative embodiments,reference is made to the accompanying figures of the drawing which forma part hereof, and in which are shown, by way of illustration, specificembodiments which may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from (e.g., still falling within) the scope of the disclosurepresented hereby.

Exemplary methods, devices, and systems shall be described withreference to FIGS. 1-7. It will be apparent to one skilled in the artthat elements or processes from one embodiment may be used incombination with elements or processes of the other embodiments, andthat the possible embodiments of such methods, devices, and systemsusing combinations of features set forth herein is not limited to thespecific embodiments shown in the Figures and/or described herein.Further, it will be recognized that the embodiments described herein mayinclude many elements that are not necessarily shown to scale. Stillfurther, it will be recognized that timing of the processes and the sizeand shape of various elements herein may be modified but still fallwithin the scope of the present disclosure, although certain timings,one or more shapes and/or sizes, or types of elements, may beadvantageous over others.

Generally, diagnostic methods and devices that can perform abeat-to-beat adjustment or modification of an AV delay, withoutinterruption of pacing therapy, are described herein. For example,exemplary devices and methods described herein relate to monitoring oneor more ventricular activation times (e.g., right ventricular activationtimes and/or left ventricular activation times) and modifying oradjusting the AV delay based on the monitored ventricular activationtimes. More specifically, in one or more embodiments, exemplary devicesand methods may monitor ventricular activation times between thedelivery of pacing stimulus to one of either the left ventricle or rightventricle using at least one pacing electrode and the sensing ofelectrical activity (e.g., depolarization) in the opposite ventricleusing one or more sensing electrodes resulting either from the pacingstimulus or an intrinsic conduction. These monitored ventricularactivation times may be used to adjust, or modify, the AV delay (e.g.,used to determine when to deliver left or right ventricular pacing) to,e.g., maintain effective pacing therapy.

One or more exemplary methods and devices described herein may providefrequent adjustment of left ventricular (LV) pace timing withoutperiodic withholding of pacing therapy (e.g., CRT) to measure apatient's intrinsic AV conduction time. In at least one embodiment, afar-field or near-field right ventricular electrogram (EGM) may bemonitored close to the timing of a LV pace. For the far-field EGM, thepeak negative slope of the EGM may indicate local right ventricular (RV)activation. For the near-field EGM, the absolute peak EGM amplitude mayindicate local RV activation. The timing of the local RV activationrelative to the delivery of the LV pace, which is the measured rightventricular activation time, may be used to adjust future LV pacingtiming (e.g., the AV delay).

Further, one or more exemplary methods and devices described herein mayallow for more frequent assessment of the need for adjustments to AVdelay for LV-only pacing without withholding or interrupting CRT pacing.More frequent adjustments of AV delay may lead to more robust fusionpacing and improved CRT response. Further, the exemplary methods anddevices may also avoid inappropriate changes in timing of LV-only pacingdue to ectopy or premature beats.

FIG. 1 is a conceptual diagram illustrating an exemplary therapy system10 that may be used to deliver pacing therapy to a patient 14. Patient14 may, but not necessarily, be a human. The therapy system 10 mayinclude an implantable medical device 16 (IMD), which may be coupled toleads 18, 20, 22 and/or a programmer 24. The IMD 16 may be, e.g., animplantable pacemaker, cardioverter, and/or defibrillator, that provideselectrical signals to the heart 12 of the patient 14 via electrodescoupled to one or more of the leads 18, 20, 22.

The leads 18, 20, 22 extend into the heart 12 of the patient 14 to senseelectrical activity of the heart 12 and/or to deliver electricalstimulation to the heart 12. In the example shown in FIG. 1, the rightventricular (RV) lead 18 extends through one or more veins (not shown),the superior vena cava (not shown), and the right atrium 26, and intothe right ventricle 28. The left ventricular (LV) coronary sinus lead 20extends through one or more veins, the vena cava, the right atrium 26,and into the coronary sinus 30 to a region adjacent to the free wall ofthe left ventricle 32 of the heart 12. The right atrial (RA) lead 22extends through one or more veins and the vena cava, and into the rightatrium 26 of the heart 12.

The IMD 16 may sense, among other things, electrical signals attendantto the depolarization and repolarization of the heart 12 via electrodescoupled to at least one of the leads 18, 20, 22. In some examples, theIMD 16 provides pacing therapy (e.g., pacing pulses) to the heart 12based on the electrical signals sensed within the heart 12. The IMD 16may be operable to adjust one or more parameters associated with thepacing therapy such as, e.g., AV delay and other various timings, pulsewide, amplitude, voltage, burst length, etc. Further, the IMD 16 may beoperable to use various electrode configurations to deliver pacingtherapy, which may be unipolar or bipolar. The IMD 16 may also providedefibrillation therapy and/or cardioversion therapy via electrodeslocated on at least one of the leads 18, 20, 22. Further, the IMD 16 maydetect arrhythmia of the heart 12, such as fibrillation of theventricles 28, 32, and deliver defibrillation therapy to the heart 12 inthe form of electrical pulses.

In some examples, a programmer 24, which may be a handheld computingdevice or a computer workstation, may be used by a user, such as aphysician, technician, another clinician, and/or patient, to communicatewith the IMD 16 (e.g., to program the IMD 16). For example, the user mayinteract with the programmer 24 to retrieve information concerning oneor more measured ventricular activation times and/or one or more pacingparameters such as AV delay (or, e.g., VV delay, modulation of multisitepacing such as pacing from more than one site in the LV or RV, etc.).The IMD 16 and the programmer 24 may communicate via wirelesscommunication using any techniques known in the art. Examples ofcommunication techniques may include, e.g., low frequency orradiofrequency (RF) telemetry, but other techniques are alsocontemplated.

FIG. 2 is a conceptual diagram illustrating the IMD 16 and the leads 18,20, 22 of therapy system 10 of FIG. 1 in more detail. The leads 18, 20,22 may be electrically coupled to a therapy delivery module (e.g., fordelivery of pacing therapy), a sensing module (e.g., one or moreelectrodes to sense or monitor electrical activity of the heart 12 foruse in determining effectiveness of pacing therapy), and/or any othermodules of the IMD 16 via a connector block 34. In some examples, theproximal ends of the leads 18, 20, 22 may include electrical contactsthat electrically couple to respective electrical contacts within theconnector block 34 of the IMD 16. In addition, in some examples, theleads 18, 20, 22 may be mechanically coupled to the connector block 34with the aid of set screws, connection pins, or another suitablemechanical coupling mechanism.

Each of the leads 18, 20, 22 includes an elongated insulative lead body,which may carry a number of conductors (e.g., concentric coiledconductors, straight conductors, etc.) separated from one another byinsulation (e.g., tubular insulative sheaths). In the illustratedexample, bipolar electrodes 40, 42 are located proximate to a distal endof the lead 18. In addition, the bipolar electrodes 44, 46 are locatedproximate to a distal end of the lead 20 and the bipolar electrodes 48,50 are located proximate to a distal end of the lead 22.

The electrodes 40, 44, 48 may take the form of ring electrodes, and theelectrodes 42, 46, 50 may take the form of extendable helix tipelectrodes mounted retractably within the insulative electrode heads 52,54, 56, respectively. Each of the electrodes 40, 42, 44, 46, 48, 50 maybe electrically coupled to a respective one of the conductors (e.g.,coiled and/or straight) within the lead body of its associated lead 18,20, 22, and thereby coupled to respective ones of the electricalcontacts on the proximal end of the leads 18, 20, 22.

The electrodes 40, 42, 44, 46, 48, 50 may further be used to senseelectrical signals attendant to the depolarization and repolarization ofthe heart 12. The electrical signals are conducted to the IMD 16 via therespective leads 18, 20, 22. In some examples, the IMD 16 may alsodeliver pacing pulses via the electrodes 40, 42, 44, 46, 48, 50 to causedepolarization of cardiac tissue of the patient's heart 12. In someexamples, as illustrated in FIG. 2, the IMD 16 includes one or morehousing electrodes, such as housing electrode 58, which may be formedintegrally with an outer surface of a housing 60 (e.g.,hermetically-sealed housing) of the IMD 16 or otherwise coupled to thehousing 60. Any of the electrodes 40, 42, 44, 46, 48 and 50 may be usedfor unipolar sensing or pacing in combination with housing electrode 58.Further, any of electrodes 40, 42, 44, 46, 48, 50, 58, which are notbeing used to deliver pacing therapy, may be used to sense electricalactivity during pacing therapy (e.g., for use in determining activationtimes). As described in further detail with reference to FIG. 3, thehousing 60 may enclose a therapy delivery module that may include astimulation generator for generating cardiac pacing pulses anddefibrillation or cardioversion shocks, as well as a sensing module formonitoring the patient's heart rhythm.

The leads 18, 20, 22 may also include elongated electrodes 62, 64, 66,respectively, which may take the form of a coil. The IMD 16 may deliverdefibrillation shocks to the heart 12 via any combination of theelongated electrodes 62, 64, 66 and the housing electrode 58. Theelectrodes 58, 62, 64, 66 may also be used to deliver cardioversionpulses to the heart 12. Further, the electrodes 62, 64, 66 may befabricated from any suitable electrically conductive material, such as,but not limited to, platinum, platinum alloy, and/or other materialsknown to be usable in implantable defibrillation electrodes. Sinceelectrodes 62, 64, 66 are not generally configured to deliver pacingtherapy, any of electrodes 62, 64, 66 may be used to sense electricalactivity during pacing therapy (e.g., for use in determining activationtimes). In at least one embodiment, the LV elongated electrode 64 may beused to sense electrical activity of a patient's heart during thedeliver of pacing therapy.

The configuration of the exemplary therapy system 10 illustrated inFIGS. 1-2 is merely one example. In other examples, the therapy systemmay include epicardial leads and/or patch electrodes instead of or inaddition to the transvenous leads 18, 20, 22 illustrated in FIG. 1.Further, in one or more embodiments, the IMD 16 need not be implantedwithin the patient 14. For example, the IMD 16 may deliver variouscardiac therapies to the heart 12 via percutaneous leads that extendthrough the skin of the patient 14 to a variety of positions within oroutside of the heart 12. In one or more embodiments, the system 10 mayutilize wireless pacing (e.g., using energy transmission to theintracardiac pacing component(s) via ultrasound, inductive coupling, RF,etc.) and sensing cardiac activation using electrodes on the can/housingand/or on subcutaneous leads.

In other examples of therapy systems that provide electrical stimulationtherapy to the heart 12, such therapy systems may include any suitablenumber of leads coupled to the IMD 16, and each of the leads may extendto any location within or proximate to the heart 12. For example, otherexamples of therapy systems may include three transvenous leads locatedas illustrated in FIGS. 1-2. Still further, other therapy systems mayinclude a single lead that extends from the IMD 16 into the right atrium26 or the right ventricle 28, or two leads that extend into a respectiveone of the right atrium 26 and the right ventricle 28.

FIG. 3 is a functional block diagram of one exemplary configuration ofthe IMD 16. As shown, the IMD 16 may include a control module 81, atherapy delivery module 84 (e.g., which may include a stimulationgenerator), a sensing module 86, and a power source 90.

The control module 81 may include a processor 80, memory 82, and atelemetry module 88. The memory 82 may include computer-readableinstructions that, when executed, e.g., by the processor 80, cause theIMD 16 and/or the control module 81 to perform various functionsattributed to the IMD 16 and/or the control module 81 described herein.Further, the memory 82 may include any volatile, non-volatile, magnetic,optical, and/or electrical media, such as a random access memory (RAM),read-only memory (ROM), non-volatile RAM (NVRAM), electrically-erasableprogrammable ROM (EEPROM), flash memory, and/or any other digital media.

The processor 80 of the control module 81 may include any one or more ofa microprocessor, a controller, a digital signal processor (DSP), anapplication specific integrated circuit (ASIC), a field-programmablegate array (FPGA), and/or equivalent discrete or integrated logiccircuitry. In some examples, the processor 80 may include multiplecomponents, such as any combination of one or more microprocessors, oneor more controllers, one or more DSPs, one or more ASICs, and/or one ormore FPGAs, as well as other discrete or integrated logic circuitry. Thefunctions attributed to the processor 80 herein may be embodied assoftware, firmware, hardware, or any combination thereof.

The control module 81 may control the therapy delivery module 84 todeliver therapy (e.g., electrical stimulation therapy such as pacing) tothe heart 12 according to a selected one or more therapy programs, whichmay be stored in the memory 82. More, specifically, the control module81 (e.g., the processor 80) may control various parameters of theelectrical stimulus delivered by the therapy delivery module 84 such as,e.g., AV delays, pacing pulses with the amplitudes, pulse widths,frequency, or electrode polarities, etc., which may be specified by oneor more selected therapy programs (e.g., AV delay adjustment programs,pacing therapy programs, pacing recovery programs, capture managementprograms, etc.). As shown, the therapy delivery module 84 iselectrically coupled to electrodes 40, 42, 44, 46, 48, 50, 58, 62, 64,66, e.g., via conductors of the respective lead 18, 20, 22, or, in thecase of housing electrode 58, via an electrical conductor disposedwithin housing 60 of IMD 16. Therapy delivery module 84 may beconfigured to generate and deliver electrical stimulation therapy suchas pacing therapy to the heart 12 using one or more of the electrodes40, 42, 44, 46, 48, 50, 58, 62, 64, 66.

For example, therapy delivery module 84 may deliver pacing stimulus(e.g., pacing pulses) via ring electrodes 40, 44, 48 coupled to leads18, 20, and 22, respectively, and/or helical tip electrodes 42, 46, and50 of leads 18, 20, and 22, respectively. Further, for example, therapydelivery module 84 may deliver defibrillation shocks to heart 12 via atleast two of electrodes 58, 62, 64, 66. In some examples, therapydelivery module 84 may be configured to deliver pacing, cardioversion,or defibrillation stimulation in the form of electrical pulses. In otherexamples, therapy delivery module 84 may be configured deliver one ormore of these types of stimulation in the form of other signals, such assine waves, square waves, and/or other substantially continuous timesignals.

The IMD 16 may further include a switch module 85 and the control module81 (e.g., the processor 80) may use the switch module 85 to select,e.g., via a data/address bus, which of the available electrodes are usedto deliver therapy such as pacing pulses for pacing therapy, or which ofthe available electrodes are used for sensing. The switch module 85 mayinclude a switch array, switch matrix, multiplexer, or any other type ofswitching device suitable to selectively couple the sensing module 86and/or the therapy delivery module 84 to one or more selectedelectrodes. More specifically, the therapy delivery module 84 mayinclude a plurality of pacing output circuits. Each pacing outputcircuit of the plurality of pacing output circuits may be selectivelycoupled, e.g., using the switch module 85, to one or more of theelectrodes 40, 42, 44, 46, 48, 50, 58, 62, 64, 66 (e.g., a pair ofelectrodes for delivery of therapy to a pacing vector). In other words,each electrode can be selectively coupled to one of the pacing outputcircuits of the therapy delivery module using the switching module 85.

The sensing module 86 is coupled (e.g., electrically coupled) to sensingapparatus, which may include, among additional sensing apparatus, theelectrodes 40, 42, 44, 46, 48, 50, 58, 62, 64, 66 to monitor electricalactivity of the heart 12, e.g., electrocardiogram (ECG)/electrogram(EGM) signals, etc. The ECG/EGM signals may be used to measure ormonitor activation times (e.g., ventricular activations times, etc.),heart rate (HR), heart rate variability (HRV), heart rate turbulence(HRT), deceleration/acceleration capacity, deceleration sequenceincidence, T-wave alternans (TWA), P-wave to P-wave intervals (alsoreferred to as the P-P intervals or A-A intervals), R-wave to R-waveintervals (also referred to as the R-R intervals or V-V intervals),P-wave to QRS complex intervals (also referred to as the P-R intervals,A-V intervals, or P-Q intervals), QRS-complex morphology, ST segment(i.e., the segment that connects the QRS complex and the T-wave), T-wavechanges, QT intervals, electrical vectors, etc.

The switch module 85 may be also be used with the sensing module 86 toselect which of the available electrodes are used to, e.g., senseelectrical activity of the patient's heart. In some examples, thecontrol module 81 may select the electrodes that function as sensingelectrodes via the switch module within the sensing module 86, e.g., byproviding signals via a data/address bus. In some examples, the sensingmodule 86 may include one or more sensing channels, each of which mayinclude an amplifier.

Signals from the selected sensing electrodes may be provided to amultiplexer, and thereafter converted to multi-bit digital signals by ananalog-to-digital converter for storage in memory 82, e.g., as anelectrogram (EGM). In some examples, the storage of such EGMs in memory82 may be under the control of a direct memory access circuit. Thecontrol module 81 (e.g., using the processor 80) may employ digitalsignal analysis techniques to analyze the digitized signals stored inmemory 82 to detect and measure, e.g., ventricular activation timeswithin the EGMs. For example, the processor 80 may be configured tomeasure activation times of cardiac tissue using EGMs from one or moreelectrodes in contact, or in proximity, with cardiac tissue by employingany of the numerous signal processing methodologies known in the art.

If IMD 16 is configured to generate and deliver pacing pulses to theheart 12, the control module 81 may include a pacer timing and controlmodule, which may be embodied as hardware, firmware, software, or anycombination thereof. The pacer timing and control module may include oneor more dedicated hardware circuits, such as an ASIC, separate from theprocessor 80, such as a microprocessor, and/or a software moduleexecuted by a component of processor 80, which may be a microprocessoror ASIC. The pacer timing and control module may include programmablecounters which control the basic time intervals associated with DDD,VVI, DVI, VDD, AAI, DDI, DDDR, VVIR, DVIR, VDDR, AAIR, DDIR and othermodes of single and dual chamber pacing. In the aforementioned pacingmodes, “D” may indicate dual chamber, “V” may indicate a ventricle, “I”may indicate inhibited pacing (e.g., no pacing), and “A” may indicate anatrium. The first letter in the pacing mode may indicate the chamberthat is paced, the second letter may indicate the chamber in which anelectrical signal is sensed, and the third letter may indicate thechamber in which the response to sensing is provided.

During pacing, escape interval counters within the pacer timing/controlmodule may be reset upon sensing of R-waves and P-waves. Therapydelivery module 84 (e.g., including a stimulation generator) may includeone or more pacing output circuits that are coupled, e.g., selectivelyby the switch module 85, to any combination of electrodes 40, 42, 44,46, 48, 50, 58, 62, or 66 appropriate for delivery of a bipolar orunipolar pacing pulse to one of the chambers of heart 12. The controlmodule 81 may reset the escape interval counters upon the generation ofpacing pulses by therapy delivery module 84, and thereby control thebasic timing of cardiac pacing functions, including anti-tachyarrhythmiapacing.

In some examples, the control module 81 may operate as an interruptdriven device, and may be responsive to interrupts from pacer timing andcontrol module, where the interrupts may correspond to the occurrencesof sensed P-waves and R-waves and the generation of cardiac pacingpulses. Any necessary mathematical calculations may be performed by theprocessor 80 and any updating of the values or intervals controlled bythe pacer timing and control module may take place following suchinterrupts. A portion of memory 82 may be configured as a plurality ofrecirculating buffers, capable of holding series of measured intervals,which may be analyzed by, e.g., the processor 80 in response to theoccurrence of a pace or sense interrupt to determine whether thepatient's heart 12 is presently exhibiting atrial or ventriculartachyarrhythmia.

The telemetry module 88 of the control module 81 may include anysuitable hardware, firmware, software, or any combination thereof forcommunicating with another device, such as the programmer 24 asdescribed herein with respect to FIG. 1. For example, under the controlof the processor 80, the telemetry module 88 may receive downlinktelemetry from and send uplink telemetry to the programmer 24 with theaid of an antenna, which may be internal and/or external. The processor80 may provide the data to be uplinked to the programmer 24 and thecontrol signals for the telemetry circuit within the telemetry module88, e.g., via an address/data bus. In some examples, the telemetrymodule 88 may provide received data to the processor 80 via amultiplexer.

The various components of the IMD 16 are further coupled to a powersource 90, which may include a rechargeable or non-rechargeable battery.A non-rechargeable battery may be selected to last for several years,while a rechargeable battery may be inductively charged from an externaldevice, e.g., on a daily or weekly basis.

Generally, one or more parameters of CRT (e.g., pacing parameters) maybe adjusted, or modified, based on one or more sensed physiologicalsignals, etc., to, e.g., deliver effective cardiac therapy to a patient.One parameter of CRT that may be adjusted is an AV delay, which may beused to determine when to deliver ventricular pacing based on eithersensed intrinsic atrial activity or paced atrial activity. Often, the AVdelay may be adjusted based on a measurement of a patient's intrinsic AVconduction time. To measure a patient's intrinsic AV conduction time,CRT methods and devices may temporarily suspend pacing therapy for oneor more heartbeats such that the natural depolarization of the patient'sheart may be monitored.

The exemplary methods and devices described herein may adjust the AVdelay without temporarily suspending pacing therapy by using ventricularactivation times monitored during pacing therapy. For example,ventricular activation times may be associated, or correlated, withoptimal timings for the delivery of ventricular pacing (e.g., LV-onlypacing, RV-only pacing, biventricular pacing, etc.), and thus,ventricular activation times monitored during pacing therapy may be usedto adjust one or more pacing parameters such as, e.g., AV delay (whichis used to determine when to deliver ventricular pacing).

Measurement of activation times can be performed by picking anappropriate fiducial point (e.g., peak or maximum values, trough orminimum values, minimum positive or negative slopes, maximum positive ornegative slopes, zero crossings, threshold crossings, etc. of a near orfar-field electrograms) and measuring a time period between the deliveryof pacing stimulus using a pacing electrode and the appropriate fiducialpoint within the electrical activity (e.g., EGM) sensed by a non-pacingelectrode. In other words, activation times between a pacing electrodeand a non-pacing electrode distant from the pacing electrode may bemeasured by picking, or selecting, an appropriate point within the EGMrecorded by the non-pacing electrode during pacing therapy (e.g.,biventricular stimulation, LV-only pacing, RV-only pacing, etc.) withrespect to the pacing spike. Exemplary activation times shown withindifferent waveforms, or EGMs, will be further described herein withrespect to FIG. 4.

Three electrograms 204, 206, 208, an atrial sense 200, and a leftventricular pace 202 are depicted over time in FIG. 4. The electrogram204 is a general purpose electrocardiogram recorded from an externalelectrode, the electrogram 206 is a bipolar, near-field electrogramusing a RV tip electrode and RV ring electrode to capture near-field RVactivity, and the electrogram 208 is a unipolar, far-field electrogramusing a RV tip electrode and, e.g., a can or housing electrode, tocapture far-field RV activity.

The peak value of the near-field electrogram 206 and the maximumnegative slope of the far-field electrogram 208 are indicative of rightventricular activation (as shown by the dashed line 210 extendingtherethrough). Thus, a RV activation time 212 may be measured betweenthe left ventricular pace 202 and one or both of the peak value of thenear-field electrogram 206 and the maximum negative slope of thefar-field electrogram 208.

The exemplary methods and/or devices described herein may track, ormonitor, ventricular activation times (e.g., right ventricularactivation time, left ventricular activation time, etc.) and adjust oneor more pacing parameters such as AV delay based on the monitoredactivation times. One manifestation of the basic flow can be seen inexemplary method 100 of FIG. 5. Exemplary method 100 includes variousprocesses to measure ventricular activation times and to modify AVdelays for use in delivering pacing therapy based on the measuredventricular activation times. Exemplary method 100 is intended toillustrate the general functional operation of the devices describedherein, and should not be construed as reflective of a specific form ofsoftware or hardware necessary to practice all of the methods describedherein. It is believed that the particular form of software will bedetermined primarily by the particular system architecture employed inthe device (e.g., IMD 16) and by the particular detection and therapydelivery methodologies employed by the device and/or system. Providingsoftware and/or hardware to accomplish the described methods in thecontext of any modern IMD, given the disclosure herein, is within theabilities of one of skill in the art.

The exemplary method 100 of FIG. 5 includes delivering pacing therapy102 (e.g., using the IMD 16 described herein). Delivering pacing therapy102 may include monitoring a patient's heart and delivering electricalpacing pulses to the patient's heart, e.g., to maintain the patient'sheartbeat (e.g., to regulate a patient's heartbeat, to improve and/ormaintain a patient's hemodynamic efficiency, etc.). More specifically,the pacing therapy 102 may include LV-only pacing or RV-only pacing. Inother words, pacing therapy may be delivered to either the leftventricle or the right ventricle of the patient's heart. As describedherein, the delivery of pacing therapy may be based on one or morepacing parameters including an AV delay, which may be modified accordingto exemplary method 100.

During the delivery of pacing therapy 102, the pacing therapy 102 maybecome less effective due to, e.g., changes in the patient's physicalactivity, changes in cardiac tissue, changes in ventricular conductionvelocity, changes in ventricular conduction patterns, changes inintrinsic conduction AV times, changes in heart rate, changes insympathetic or parasympathetic stimulation, etc. To compensate for suchchanges, the exemplary method 100 may sense electrical activity of thepatient's heart during the delivery of pacing therapy 104 with one ormore electrodes not used to pace the patient's heart. For example, theelectrical activity may be sensed 104, or monitored, using at least onesensing electrode during the delivery of pacing stimulus (e.g., pacingpulses) from one or more pacing electrodes such that the electricalactivity which results from the pacing stimulus or an intrinsicconduction (e.g., whichever occurs first and results in adepolarization) may be sensed, or seen, in the electrogram of the atleast one sensing electrode (e.g., unipolar or bipolar configurations).

More specifically, ventricular activation times may be measured 106using the sensed electrical activity during the delivery of pacingtherapy 104. In at least one embodiment, the pacing therapy may beLV-only pacing therapy, which includes pacing stimulus delivered to onlythe left ventricle of the patient's heart (and not the right ventricle).As such, the exemplary method delivering LV-only pacing therapy maymeasure the right ventricular activation time, which is the time betweenthe delivery of pacing stimulus to the left ventricle and thedepolarization of the right ventricle due to, e.g., an intrinsicconduction or the pacing stimulus delivered to the left ventricle andconducted to the right ventricle (e.g., whichever occurs first).

In at least another embodiment, the pacing therapy may be RV-only pacingtherapy, which includes pacing stimulus delivered to only the rightventricle of the patient's heart (and not the left ventricle). As such,the exemplary method delivering RV-only pacing therapy may measure theleft ventricular activation time, which is the time between the deliveryof pacing stimulus to the right ventricle and the depolarization of theleft ventricle due to, e.g., an intrinsic conduction or the pacingstimulus to the right ventricle (whichever occurs first).

The electrodes used to deliver pacing stimulus and sensing electricalactivity for use in measuring activation times may be described in termsof a first electrode and a second electrode. For example, the pacingtherapy may be delivered with at least a first electrode and theelectrical activity may be sensed with at least a second electrode. Thesecond electrode, or any other electrode configured to sense theelectrical activity during the delivery of pacing stimulus, may not be apacing electrode. In other words, the second electrode may not beconfigured for delivering pacing therapy. Further, the second, orsensing, electrode may be a pacing electrode that is simply not beingused to deliver pacing therapy (e.g., not ever used for delivery ofpacing therapy, not being used to deliver pacing therapy at the sametime as the first electrode, etc.).

In the example of LV-only pacing therapy, the first electrode, or thepacing electrode, may be configured to pace the left ventricle ofpatient's heart and the second electrode, or the sensing electrode, maybe configured to sense electrical activity of the right ventricle ofpatient's heart. In at least one LV-only pacing therapy embodiment, thepacing electrode, or first electrode, may be a LV tip electrodeconfigured to pace the free wall of the left ventricle, and the sensingelectrode, or second electrode, may be a RV ring electrode, a RV tipelectrode, or a RV elongated (e.g., defibrillation) electrode. Forexample, a near-field right ventricular EGM may be measured, or sensed,between a RV tip electrode and a RV ring electrode. Further, forexample, a far-field right ventricular EGM may be measured, or sensed,between a RV tip electrode and a can, or housing, electrode (e.g.,electrode 58).

In the example of RV-only pacing therapy, the first electrode, or thepacing electrode, may be configured to pace the right ventricle ofpatient's heart and the second electrode, or the sensing electrode, maybe configured to sense electrical activity of the left ventricle ofpatient's heart. In at least one RV-only pacing therapy embodiment, thepacing electrode, or first electrode, may be a RV tip electrodeconfigured to pace the endocardial apical or septal wall of the rightventricle, and the sensing electrode, or second electrode, may be a LVring electrode, a LV tip electrode, or a LV elongated (e.g.,defibrillation) electrode. For example, a near-field left ventricularEGM may be measured, or sensed, between a LV tip electrode and a LV ringelectrode. Further, for example, a far-field left ventricular EGM may bemeasured, or sensed, between a LV tip electrode and a can, or housing,electrode (e.g., electrode 58).

Further, the second electrode may be located a distance away from thefirst electrode (e.g., the pacing electrode) such that an activationtime may be monitored. For example, if the first electrode and thesecond electrode were located too close to one another, an activationtime may be too short for use in modifying one more pacing parameterssuch as, e.g., AV delay.

Since the exemplary method 100 may not use pacing electrodes to sensethe electrical activity for use in modifying one or more pacingparameters such as AV delay, sensing 104 may take place for every pacedbeat (e.g., beat-to-beat, etc.) such that pacing therapy is notinterrupted. As such, the ventricular activation time for each heartbeatof a plurality of heartbeats may be monitored using exemplary method 100without interruption.

As described herein, ventricular activation times may be measured 106using the electrical activity sensed 104 using one or more sensingelectrodes. For example, a fiducial point on the waveform of the sensedelectrical activity may be selected to be used with the exemplary method100. The time between the delivery of the pacing therapy (e.g., to theLV, to the RV, etc.) and the selected fiducial point within the waveformof the sensed electrical activity resulting from either the pacingtherapy (e.g., pacing stimulus) or an intrinsic activation is themeasured ventricular activation time.

The fiducial point may be selected to be a characteristic of the sensedelectrical activity resulting from (e.g., a product of) the pacingtherapy or an intrinsic activation that is repeatedly, or consistently,recognizable such that ventricular activation times may be repeatedly,or consistently, measured. In at least one embodiment, the fiducialpoint may be a peak, or maximum, value in a near-field waveform, orelectrogram, sensed by the sensing electrode (e.g., as shown byelectrogram 206 shown in FIG. 4). In at least another embodiment, thefiducial point may be a peak, or maximum, negative slope value (e.g.,the steepest negative slope) in a far-field waveform, or electrogram,sensed by the sensing electrode (e.g., as shown by electrogram 208 shownin FIG. 4). For example, a derivative function of the far-fieldelectrogram may be calculated, or computed, to determine the peak, ormaximum, negative slope value of the far-field electrogram.

Based on the measured activation times 106, the exemplary method 100 maymodify an AV delay, e.g., which may be used in the delivery of pacingtherapy, based on the measured activation times 108. As shown in FIG. 5,before the exemplary method 100 may modify the AV delay 110, theexemplary method 100 may evaluate one or more of the measured activationtimes 108 to determine whether the AV delay should be modified. Forexample, one or more measured activation time may be compared to apredetermined reference activation time indicative of effective, oroptimal, pacing therapy. If the one or more activation times are greaterthan or less than the predetermined reference activation time by aselected threshold value, then exemplary method 100 may modify the AVdelay.

The predetermined reference activation time may be defined as an optimalvalue of the ventricular activation time that produces a maximum strokevolume for a fixed heart rate or the maximum cardiac output for a sinusnode driven heart rate or provides effective hemodynamic performance asmeasured by any number of invasive or noninvasive methods, includingsensor-based measurements, narrowest QRS duration on the surface ECG,etc. In at least one embodiment, the predetermined reference activationtime may be determined by a mapping function implemented within animplantable medical device which may determine the optimal value basedon assessment of patient's intrinsic AV conduction at rest.

The predetermined reference activation time may be established by anynumber of methods. In at least one embodiment, a mean valuerepresentative of a plurality of monitored ventricular activation times,and its variability, may be evaluated for a selected number of beats(e.g., 60 heartbeats) during a post-implant follow-up at physician'soffice. A physician, or another practitioner, may visually monitor thepacing therapy using, e.g., an electrocardiogram of the patient's heart,to confirm that the pacing therapy is effective. Then, for example, themean value (or any other statistic) of the monitored ventricularactivation times may be calculated and used as the predeterminedreference activation time indicative of effective pacing therapy. Thevariability may be used to calculate a threshold value, which is thedifference from the reference activation time that may be acceptable orallowable without indicating ineffective pacing therapy (which mayinitiate an AV delay adjustment).

In at least one embodiment, a reference activation time for LV-onlypacing may be about 50 milliseconds (ms) and the threshold may be about10 ms. In other words, in this embodiment, if the measured activationtime is between about 40 ms (i.e., 50 ms minus 10 ms) and about 60 ms(i.e., 50 ms plus 10 ms), then the measured activation time may bedetermined to provide effective (e.g., optimal) pacing therapy, andthus, no modification of the AV delay should occur. Such values may bedifferent for each patient and each different electrode combination usedfor pacing and sensing.

The predetermined reference activation time may be greater than or equalto about 30 ms, about 35 ms, about 40 ms, about 50 ms, about 55 ms, etc.Further, the predetermined reference activation time may be less than orequal to about 60 ms, about 65 ms, about 70 ms, about 80 ms, etc. Thethreshold value may be greater than or equal to about 2 ms, about 3 ms,about 5 ms, about 7 ms, etc. Further, the threshold value may be lessthan or equal to about 10 ms, about 12 ms, about 15 ms, 17 ms, etc.

In at least one embodiment, a single measured ventricular activationtime may be evaluated 108 to initiate, or trigger, the modification ofan AV delay 110. For example, if the last measured ventricularactivation time is greater than or less than a predetermined referenceactivation time by a selected threshold value, then it may be determined108 that the AV delay should be modified 110.

In other embodiments, more than one measured ventricular activation timemay be evaluated 108 to initiate the modification of an AV delay 110.For example, if a first selected number (e.g., 5, 10, etc.) ofconsecutive measured ventricular activation times are greater than orless than a predetermined reference activation time by a selectedthreshold value over a second selected number (e.g., 10, 20, etc.) ofheartbeats, then it may be determined that the AV delay should bemodified 110. An exemplary method including a determination processbased on more than one measured activation time to modify an AV delay isfurther described herein with reference to FIG. 6.

Still further, the standard deviation of one or more measuredventricular activation times may be compared to a selected variabilitythreshold value to determine 108 if the AV delay should be modified 110.For example, if the standard deviation of a first selected number (e.g.,5) of the activation time differences (e.g., an activation timedifference may be the measured ventricular activation time minus thepredetermined reference activation time) are less than a selectedvariability threshold (e.g., 15 ms) over a second selected number ofheartbeats (e.g., 10 heartbeats), then it may be determined that the AVdelay should be modified 110. An exemplary method including adetermination process based on the standard deviation of one or moremeasured ventricular activation times to modify an AV delay is furtherdescribed herein with reference to FIG. 6.

Although standard deviation is used in this example to evaluatevariability, any statistical metric may be used to evaluate the measuredactivation times. For example, the mean of absolute deviations of eachactivation time from the mean activation time may be used to evaluatethe measured activation times.

If it is determined the AV delay should not be adjusted or modified, theexemplary method 100 may continue delivering pacing therapy 102, sensingelectrical activity 104, measuring ventricular activation times 106, andevaluating the activation times 108 to determine whether AV delay or anyother pacing parameter (e.g., VV delay, modulation of multisite pacingsuch as pacing from more than one site in the LV or RV, etc.) should beadjusted. If it is determined that AV delay should be adjusted ormodified 108, the exemplary method 100 may advance to modifying the AVdelay 110.

Generally, to modify the AV delay 110, the AV delay may be shortened orlengthened based upon the monitored ventricular activation times. Forexample, in LV-only pacing (e.g., in patients with left bundle branchblock), if the ventricular activation times (e.g., the time periodbetween the LV pace and the RV activation such as RV activation time 212shown in FIG. 4) increase during subsequent fusion resynchronizationpacing (e.g., RV activation occurs after LV pacing, moving toward aright bundle branch block pattern of activation, as shown in example 310of FIG. 7 further described below), the AV delay for the LV pacing maybe lengthened, or adjusted to longer values, to maintain ideal fusion.Conversely, in LV-only pacing, if the ventricular activation timessubsequently decrease or become negative (e.g., RV activation occursbefore LV pacing, moving toward a left bundle branch block pattern ofactivation, as shown in example 320 of FIG. 7 further described below),the AV delay for pacing may be shortened, or adjusted to shorter values,to maintain ideal fusion.

Further, when modifying the AV delay 110, either the last used AV delayor one or more statistical variations of one or more previously used AVdelays may be used as a starting point for modification. In at least oneembodiment, the AV delay may be set to the last used AV delay plus thelast measured activation time minus the predetermined referenceactivation time.

In one or more embodiments, a mode, median, or average of a selectednumber of previous AV delays (e.g., the last five AV different delays,the AV delays used for the last 5 heartbeats, 10 heartbeats, 15heartbeats, 20 heartbeats, etc.) may be used to modify the AV delay. Forexample, the AV delay may be set to one of a mode, a median, or averageof a selected number (e.g., 5, 10 etc.) of previous AV delays plus oneof a mode, a median, or average of the selected number of activationtime differences. As described herein, an activation time difference maybe the measured ventricular activation time minus the predeterminedreference activation time.

Another exemplary method 150 for use in modifying AV delay based onmonitored ventricular activation times is depicted in FIG. 6. Exemplarymethod 150 is configured to deliver LV-only pacing therapy 152. As such,the ventricular activation times measured in exemplary method 150 arethe right ventricular activation times since the left ventricle is beingpaced. In other words, the measured activation time is the time periodbetween the delivery of a left ventricular pace and the depolarizationof the right ventricle due to, e.g., either the pacing stimulus to theleft ventricle or an intrinsic conduction.

For each paced heartbeat 152, a measured activation time, or ΔT, may bemeasured and calculated continuously or periodically for a selectednumber of, or N, heartbeats 154 (e.g., 10 successive heartbeats). Themeasured activation time, or ΔT, is equal to the time between an atrialsense or atrial pace and the right ventricular activation (e.g., asdetermined by analysis of an electrogram of the right ventricle) minusthe AV delay (e.g., the time between either an atrial sense or atrialpace and the left ventricular pace).

The measured activation times may then be evaluated 156. For example, ifthe measured activation time, or ΔT, is less than (e.g., less than orequal to) a predetermined reference activation time, or ΔTref, by aselected threshold value or is greater than (e.g., greater than or equalto) the predetermined reference activation time, or ΔTref, by theselected threshold, then it may be determined 156 that the AV delayshould be adjusted 158.

As shown, the determination process 156 may also evaluate anystatistical metric of the measured activation times such as, e.g., thestandard deviation of the measured activation times from thepredetermined reference activation times. For example, if a standarddeviation of the difference between the measured activation time, or ΔT,and the predetermined reference activation time, or ΔTref, is less thana variability limit (e.g., 15 ms) for a first selected number of, or M,beats (e.g., 5 heartbeats) out a second selected number of, or N, beats(e.g., 10 heartbeats), then it may be determined 156 that the AV delayshould be adjusted 158. If it is not determined that AV delay should beadjusted in process 156, the exemplary method 150 may continuedelivering LV pacing 152 without adjusting the AV delay.

If it is determined that the AV delay should be adjusted, then the AVdelay (e.g., the time period between the atrial sense or atrial pace andthe ventricular pace) may be adjusted for the next beat 158. Forexample, the AV delay may be set to the median or modal value of the AVdelay of a selected number of, or M, heartbeats (e.g., 5 heartbeats)plus a median or modal value of the difference between the measuredactivation times, or ΔT, and the predetermined reference activationtime, or ΔTref, of the selected number of, or M, heartbeats 158.

After the AV delay has been adjusted 158, the exemplary method 150 mayevaluate the newly adjusted AV delay 160 to, e.g., determine if the AVdelay has been adjusted too far to deliver effective pacing therapy. Forexample, if the AV delay for pacing reaches an upper limit, the LV-onlyfusion pacing may be replaced by biventricular pacing at a shortpredetermined AV delay 162. The short predetermined AV delay may beabout 80 ms to about 200 ms. If the AV delay has not exceeded apredetermined upper limit value, then exemplary method 150 may continuedelivering LV-only pacing 152, e.g., using the adjusted AV delay,monitoring ventricular activations times 154, and evaluating themeasured ventricular activation times 156.

Further, although exemplary method 150 is shown for LV-only pacing, itmay also be used for patients with right bundle branch block by pacingRV-only and sensing left ventricle activation to determine the AV delayfor RV pacing.

Three exemplary calculations of measured activation times, or ΔT, areshown in FIG. 7. In these examples, the predetermined referenceactivation time, or ΔTref, for optimal fusion pacing is 50 ms and thethreshold value is 10 ms.

In the first example 300, the time period between an atrial sense (As)and a ventricular pace (VP) is 140 ms (which is the AV delay) and thetime period between the atrial sense and a right ventricular activationis 190 ms. Thus, the measured activation time, or ΔT, equals 50 ms,which is within the threshold value, 10 ms, of the predeterminedreference activation time of 50 ms. Therefore, no adjustment to the AVdelay may be triggered or initiated.

In the second example 310, the time period between an atrial sense (As)and a ventricular pace (VP) is 140 ms (which is the AV delay) and thetime period between the atrial sense (As) and right ventricularactivation is 220 ms. Thus, the measured activation time, or ΔT, equals80 ms, which is not within the threshold value, 10 ms, of thepredetermined reference activation time of 50 ms. Therefore, adjustmentof the AV delay may be initiated.

As shown, the difference between the measured activation time and thepredetermined reference activation time may be calculated, which is 30ms. The AV delay, or As-VP(next beat), may be set to the last AV delay(As-VP), which was 140 ms, plus the difference between the measuredactivation time and the predetermined reference activation time. Assuch, the new AV delay may be set to 170 ms. The dotted line in example310 depicts where the next ventricular pace (VP) should occur using thenewly adjusted AV delay.

In the third example 320, the time period between an atrial sense (As)and a ventricular pace (VP) is 140 ms (which is the AV delay) and thetime period between the atrial sense (As) and right ventricularactivation is 130 ms. In this example, the right ventricular activationoccurred before the ventricular pace (VP), and therefore, the rightventricular activation may have occurred naturally (e.g., intrinsicconduction). The measured activation time, or ΔT, equals −10 ms, whichis not within the threshold value, 10 ms, of the predetermined referenceactivation time of 50 ms. Therefore, adjustment of the AV delay may beinitiated.

As shown, the difference between the measured activation time and thepredetermined reference activation time may be calculated, which is −60ms. The AV delay, or As-VP(next beat), may be set to the last AV delay(As-VP), which was 140 ms, plus the difference between the measuredactivation time and the predetermined reference activation time. Assuch, the new AV delay may be set to 80 ms. The dotted line in example320 depicts where the next ventricular pace (VP) should occur using thenewly adjusted AV delay.

The techniques described in this disclosure, including those attributedto the IMD 16, the programmer 24, or various constituent components, maybe implemented, at least in part, in hardware, software, firmware, orany combination thereof. For example, various aspects of the techniquesmay be implemented within one or more processors, including one or moremicroprocessors, DSPs, ASICs, FPGAs, or any other equivalent integratedor discrete logic circuitry, as well as any combinations of suchcomponents, embodied in programmers, such as physician or patientprogrammers, stimulators, image processing devices, or other devices.The term “module,” “processor,” or “processing circuitry” may generallyrefer to any of the foregoing logic circuitry, alone or in combinationwith other logic circuitry, or any other equivalent circuitry.

Such hardware, software, and/or firmware may be implemented within thesame device or within separate devices to support the various operationsand functions described in this disclosure. In addition, any of thedescribed units, modules, or components may be implemented together orseparately as discrete but interoperable logic devices. Depiction ofdifferent features as modules or units is intended to highlightdifferent functional aspects and does not necessarily imply that suchmodules or units must be realized by separate hardware or softwarecomponents. Rather, functionality associated with one or more modules orunits may be performed by separate hardware or software components, orintegrated within common or separate hardware or software components.

When implemented in software, the functionality ascribed to the systems,devices and techniques described in this disclosure may be embodied asinstructions on a computer-readable medium such as RAM, ROM, NVRAM,EEPROM, FLASH memory, magnetic data storage media, optical data storagemedia, or the like. The instructions may be executed by one or moreprocessors to support one or more aspects of the functionality describedin this disclosure.

This disclosure has been provided with reference to illustrativeembodiments and is not meant to be construed in a limiting sense. Asdescribed previously, one skilled in the art will recognize that othervarious illustrative applications may use the techniques as describedherein to take advantage of the beneficial characteristics of theapparatus and methods described herein. Various modifications of theillustrative embodiments, as well as additional embodiments of thedisclosure, will be apparent upon reference to this description.

1. An implantable medical device operable for delivery of cardiactherapy to a patient, the device comprising: a therapy delivery moduleconfigured to deliver pacing therapy to either the left ventricle or theright ventricle of a patient's heart using at least one electrode; asensing module configured to sense electrical activity of the patient'sheart using at least one other electrode; and a control module coupledto the therapy delivery module and to the sensing module and configuredto: determine an AV delay, control the delivery of the pacing therapy toeither the left ventricle or the right ventricle of a patient's heartbased on the AV delay, wherein the pacing therapy is delivered over aplurality of heartbeats, sense the electrical activity of the patient'sheart using the sensing module during the delivery of the pacingtherapy, measure a ventricular activation time for each of the pluralityof heartbeats between the delivery of pacing stimulus of the pacingtherapy and at least one selected fiducial point of the sensedelectrical activity resulting from at least one of the delivered pacingstimulus of the pacing therapy and an intrinsic conduction of thepatient's heart, modify the AV delay for use in delivering pacingtherapy based on the measured ventricular activation times in responseto determining that one or more of the measured ventricular activationtimes resulting from the delivered pacing stimulus of the pacing therapyare greater than or less than a predetermined reference activation timeby a selected threshold value, and modify the AV delay for use indelivering pacing therapy based on the measured ventricular activationtimes in response to determining that one or more of the measuredventricular activation times resulting from the intrinsic conduction ofthe patient's heart are greater than or less than a predeterminedreference activation time by a selected threshold value.
 2. The deviceof claim 1, wherein, to modify the AV delay for use in delivering pacingtherapy based on the measured ventricular activation times, the controlmodule is further configured to set the AV delay to the last AV delayplus the last measured activation time minus the predetermined referenceactivation time.
 3. The device of claim 1, wherein, to modify the AVdelay for use in delivering pacing therapy based on the measuredventricular activation times, the control module is further configuredto set the AV delay to either a mode or a median of a selected number ofprevious AV delays plus either a mode or median of a selected number ofactivation time differences, wherein the activation time differences arethe measured ventricular activation times minus the predeterminedreference activation time.
 4. The device of claim 1, wherein the controlmodule is configured to modify the AV delay for use in delivering pacingtherapy based on the measured ventricular activation times if a firstselected number of the measured ventricular activation times are greaterthan or less than the predetermined reference activation time by theselected threshold value over a second selected number of heartbeats. 5.The device of claim 1, wherein, to modify the AV delay for use indelivering pacing therapy based on the measured ventricular activationtimes, the control module is configured to modify the AV delay for usein delivering pacing therapy if a standard deviation of a first selectednumber of activation time differences are less than a selectedvariability threshold value over a second selected number of heartbeats,wherein the activation time differences are the measured ventricularactivation times minus the predetermined reference activation time. 6.The device of claim 1, wherein the sensing module is configured to senseelectrical activity of the right ventricle of the patient's heart usingthe at least one electrode, and wherein the control module is configuredto sense electrical activity of the right ventricle of the patient'sheart using the sensing module during the delivery of the pacingtherapy.
 7. The device of claim 1, wherein sensing module is furtherconfigured to sense near-field electrical activity of the rightventricle of the patient's heart using the at least one electrode,wherein the control module is configured to sense near-field electricalactivity of the right ventricle of the patient's heart using the sensingmodule during the delivery of the pacing therapy, and wherein the atleast one selected fiducial point of the sensed electrical activitycomprises a maximum value of the near-field electrical activity of theright ventricle of the patient's heart.
 8. The device of claim 1,wherein sensing module is further configured to sense far-fieldelectrical activity of the right ventricle of the patient's heart usingthe at least one electrode, wherein the control module is configured tosense far-field electrical activity of the right ventricle of thepatient's heart using the sensing module during the delivery of thepacing therapy, and wherein the at least one selected fiducial point ofthe sensed electrical activity comprises a maximum negative slope of thefar-field electrical activity of the right ventricle of the patient'sheart.
 9. The device of claim 1, wherein the at least one electrode isconfigured to deliver pacing therapy to the left ventricle of thepatient's heart.
 10. The device of claim 1, wherein the therapy deliverymodule is further configured to deliver bi-ventricular pacing therapy tothe patient's heart, and wherein the control module is furtherconfigured to initiate the delivery of bi-ventricular pacing therapy tothe patient's heart using the therapy delivery module if the modified AVdelay is greater than a selected limit value.
 11. A method for use in animplantable medical device operable for delivery of cardiac therapy to apatient, the method comprising: delivering pacing therapy to either theleft ventricle or the right ventricle of a patient's heart using atleast one electrode based on an AV delay using an implantable medicaldevice, wherein the pacing therapy is delivered over a plurality ofheartbeats; sensing electrical activity of the patient's heart using atleast one other electrode of the implantable medical device during thedelivery of the pacing therapy; measuring a ventricular activation timefor each of the plurality of heartbeats between the delivery of pacingstimulus of the pacing therapy and at least one selected fiducial pointof the sensed electrical activity resulting from at least one of thedelivered pacing stimulus of the pacing therapy and an intrinsicconduction of the patient's heart; modifying the AV delay for use indelivering pacing therapy based on the measured ventricular activationtimes in response to determining that one or more of the measuredventricular activation times resulting from the delivered pacingstimulus of the pacing therapy are greater than or less than apredetermined reference activation time by a selected threshold value;and modifying the AV delay for use in delivering pacing therapy based onthe measured ventricular activation times in response to determiningthat one or more of the measured ventricular activation times resultingfrom the intrinsic conduction of the patient's heart are greater than orless than a predetermined reference activation time by a selectedthreshold value.
 12. The method of claim 11, wherein modifying the AVdelay for use in delivering pacing therapy based on the measuredventricular activation times comprises setting the AV delay to the lastAV delay plus the last measured ventricular activation time minus thepredetermined reference activation time.
 13. The method of claim 11,wherein modifying the AV delay for use in delivering pacing therapybased on the measured ventricular activation times comprises setting theAV delay to either a mode or a median of a selected number of previousAV delays plus either a mode or median of the selected number ofventricular activation time differences, wherein the ventricularactivation time differences are the measured ventricular activationtimes minus the predetermined reference activation time.
 14. The methodof claim 11, wherein modifying the AV delay for use in delivering pacingtherapy based on the measured ventricular activation times if a firstselected number of the measured ventricular activation times are greaterthan or less than the predetermined reference activation time by theselected threshold value over a second selected number of heartbeats.15. The method of claim 11, wherein modifying the AV delay for use indelivering pacing therapy based on the measured ventricular activationtimes comprises modifying the AV delay for use in delivering pacingtherapy if a standard deviation of a first selected number of theactivation time differences are less than a selected variabilitythreshold value over a second selected number of heartbeats, wherein theventricular activation time differences are the measured ventricularactivation times minus the predetermined reference activation time. 16.The method of claim 11, wherein sensing electrical activity of thepatient's heart using at least one electrode of the implantable medicaldevice during the delivery of the pacing therapy comprises sensingelectrical activity of the right ventricle of the patient's heart usingthe at least one electrode.
 17. The method of claim 11, wherein sensingelectrical activity of the patient's heart using at least one electrodeof the implantable medical device during the delivery of the pacingtherapy comprises sensing near-field electrical activity of the rightventricle of the patient's heart using the at least one electrode duringthe delivery of the pacing therapy, and wherein the at least oneselected fiducial point of the sensed electrical activity comprises amaximum value of the near-field electrical activity of the rightventricle of the patient's heart.
 18. The method of claim 11, whereinsensing electrical activity of the patient's heart using at least oneelectrode of the implantable medical device during the delivery of thepacing therapy comprises sensing far-field electrical activity of theright ventricle of the patient's heart using the at least one electrodeduring the delivery of the pacing therapy, and wherein the at least oneselected fiducial point of the sensed electrical activity comprises amaximum negative slope of the far-field electrical activity of the rightventricle of the patient's heart.
 19. The method of claim 11, whereindelivering pacing therapy to either the left ventricle or the rightventricle of a patient's heart using at least one electrode based on anAV delay using an implantable medical device comprises delivering pacingtherapy to the left ventricle of the patient's heart.
 20. The method ofclaim 11, wherein the method further comprises delivering bi-ventricularpacing therapy to the patient's heart using the implantable medicaldevice if the modified AV delay is greater than a selected limit value.